Long-Term Effects of Low-Level Blast Exposure and High-Caliber Weapons Use in Military Special Operators

Abstract

Chronic low-level blast exposure has been linked with neurological alterations and traumatic brain injury (TBI) biomarkers. Impaired smooth-pursuit eye movements (SPEM) are often associated with TBI. The purpose of this study was to determine whether long-term operators of low-level blast exposure or high-caliber weapons use displayed oculomotor behaviors that differed from controls. Twenty-six members of an elite military unit performed a computerized oculomotor testing task using an eye tracker and completed a concussion assessment questionnaire. The participants were split into a blast exposure group and control group. The blast exposure group had a history of exposure to low-level blasts or high-caliber weapon use. The results revealed significant differences in SPEM, saccades, and fixations between the blast exposure group and control group. The blast exposure group’s eye movements were slower, stopped at more frequent points when following a target, traveled further from the target in terms of both speed and direction, and showed higher rates of variation and inefficiency. Poor oculomotor behavior correlated with a higher symptom severity on the concussion assessment questionnaire. Military special operators exposed to long-term low-level blasts or high-caliber weapons usage displayed an impaired oculomotor behavior in comparison to controls. These findings further our understanding of the impact of long-term low-level blast exposure on the oculomotor behavior of military special operators and may inform practical implications for military training.

Full text

Citation: Hunfalvay, M.; Murray,
N.P.; Creel, W.T.; Carrick, F.R.
Long-Term Effects of Low-Level Blast
Exposure and High-Caliber Weapons
Use in Military Special Operators.
Brain Sci. 2022,12, 679. https://
doi.org/10.3390/brainsci12050679
Academic Editor: Sergio Bagnato
Received: 2 April 2022
Accepted: 17 May 2022
Published: 23 May 2022
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brain
sciences
Article
Long-Term Effects of Low-Level Blast Exposure and
High-Caliber Weapons Use in Military Special Operators
Melissa Hunfalvay 1, * , Nicholas P. Murray 2, William T. Creel 3and Frederick R. Carrick 4,5,6,*
1RightEye LLC, 7979 Old Georgetown Rd, Suite 801, Bethesda, MD 20814, USA
2Department of Kinesiology, East Carolina University, Minges Coliseum 166, Greensville, NC 27858, USA;
murrayni@ecu.edu
3Neurology Department, Adler University, 17 N Dearborn St, Chicago, IL 60602, USA; wcreel@adler.edu
4Neurology Department, College of Medicine, University of Central Florida, Orlando, FL 32827, USA
5Centre for Mental Health Research in Association with University of Cambridge, Cambridge CB2 1TN, UK
6
Department of Health Professions Education, MGH Institute for Health Professions, Boston, MA 02129, USA
*Correspondence: melissa@righteye.com (M.H.); drfrcarrick@post.harvard.edu (F.R.C.)
Abstract:
Chronic low-level blast exposure has been linked with neurological alterations and trau-
matic brain injury (TBI) biomarkers. Impaired smooth-pursuit eye movements (SPEM) are often
associated with TBI. The purpose of this study was to determine whether long-term operators of low-
level blast exposure or high-caliber weapons use displayed oculomotor behaviors that differed from
controls. Twenty-six members of an elite military unit performed a computerized oculomotor testing
task using an eye tracker and completed a concussion assessment questionnaire. The participants
were split into a blast exposure group and control group. The blast exposure group had a history of
exposure to low-level blasts or high-caliber weapon use. The results revealed significant differences
in SPEM, saccades, and fixations between the blast exposure group and control group. The blast
exposure group’s eye movements were slower, stopped at more frequent points when following a
target, traveled further from the target in terms of both speed and direction, and showed higher rates
of variation and inefficiency. Poor oculomotor behavior correlated with a higher symptom severity on
the concussion assessment questionnaire. Military special operators exposed to long-term low-level
blasts or high-caliber weapons usage displayed an impaired oculomotor behavior in comparison to
controls. These findings further our understanding of the impact of long-term low-level blast expo-
sure on the oculomotor behavior of military special operators and may inform practical implications
for military training.
Keywords: SPEM; smooth-pursuit eye movements; oculomotor; TBI; military
1. Introduction
There has been a growing interest in understanding the effects of long-term low-level
blast exposure and high-caliber weapon use among military personnel due to concern over
potential adverse health outcomes [
1
]. Recent findings suggest that chronic exposure to
low-level blasts may be implicated in neurological alterations and elevated biomarkers
associated with traumatic brain injury (TBI) [2,3].
TBI is a unique and prominent cause of morbidity in military life [
4
,
5
]. Given the
prevalent nature of TBI in military populations, there is an evolving need to understand how
repeated low-level blast exposure and high-caliber weapon use may present an additional
threat to service members’ mental health and well-being. Determining the impact of low-
level blast exposure on cognitive functioning has proven especially challenging given the
evasive nature of concussive injury that is invisible to the eye. However, eye-tracking
technology offers a quantitative, non-invasive, and sensitive solution that can provide a
detailed insight into the brain and cognitive functioning of military personnel following
low-level blast exposure [6].
Brain Sci. 2022,12, 679. https://doi.org/10.3390/brainsci12050679 https://www.mdpi.com/journal/brainsci

Brain Sci. 2022,12, 679 2 of 9
Eye-tracking technology is a uniquely equipped tool for accurately and efficiently
measuring objective TBI biomarkers, including for differentiating the severity of TBI [
7
–
11
].
Eye tracking has been used to assess neurological functioning, oculomotor assessment,
neurocircuitry abnormalities, and even map oculomotor dysfunction to associated brain
regions [12–14].
Oculomotor assessments can be subdivided into discrete eye movements including
smooth pursuits, fixations, and saccades [
15
]. Smooth pursuits use predictive eye-tracking
movements to stabilize a moving target on the fovea, fixations involve maintaining a fixed
eye contact on a visual target input, and saccades are short and quick eye movements
between two points. Collectively, smooth pursuits, fixations, and saccades can reveal
neurological abnormalities through a comprehensive smooth pursuit eye movement (SPEM)
assessment.
The smooth pursuit system is very complex, and while not yet fully understood, it
is what allows humans to predictively track moving objects [
16
–
18
]. After visual inputs
are processed in the striate cortex, information is relayed to specialized neurons in the
extrastriate areas, which extend to the brainstem and allow communication with the
cerebellum [
17
]. The cerebellum is intimately involved in visual perception and is critical in
generating pursuits [
19
]. The pursuits themselves are primarily controlled by the frontal eye
field and deeper brain structures including the basal ganglia and superior colliculus [17].
Circular smooth pursuits (CSP) have been observed to activate the visual cortex
bilaterally and the right inferior parietal sulcus [
20
]. Neuron firing has also been detected
in the medial temporal lobes and premotor cortex with a marked depressed activity in the
insula and anterior cingulate [
21
]. Evidently, the complexity and breadth of the smooth
pursuit networks makes the system susceptible to damage from TBI.
CSP can be tracked to measure fixation percentages. Concussed individuals have
higher fixation percentages as they are constantly falling behind the target, requiring their
eyes to saccade to catch up to the target [
22
]. Fixation is a conscious process involving
a network of brain regions including the parietal eye field, supplemental eye field, V5
and V5A areas, and the dorsolateral prefrontal cortex [
17
]. The inability to smoothly
track moving objects causes abnormal fixations that may be indicative of damage to these
networks [9].
Variance is another metric that is obtained from tracking CSP. The neurons of the pons
contribute to variance measures, as they are tuned to the eye velocity and can be stimulated
to change the velocity of pursuits. The pontine nuclei project to the cerebellum, which
is involved in the online correction of velocity during pursuit. A high smooth pursuit
variance shows inefficiencies in eye movement that are often indicative of TBI [23].
The saccadic system includes several brain structures including the brain stem, pons,
midbrain, and cerebral cortex [
17
]. Saccades are generated by burst neuron circuits in the
brain stem, which activate motor signals that control the extraocular muscles in the eye [
17
].
Multiple studies have shown that saccadic impairment is associated with TBI [10,24].
More recent approaches to understanding the impact of low-level blast exposure on
brain health have revealed elevated biomarkers associated with traumatic brain injury and
brain diseases in military populations [
3
]. These results further validate efforts towards
identifying objective biomarkers that can inform clinically relevant diagnostic tools.
In contrast, most current methods of TBI diagnosis involve an element of subjectivity
that lacks sensitivity to intricate symptomology [
7
]. The Sport Concussion Assessment Tool
(SCAT) is among the most respected concussion inventories and is widely used by team
physicians [
25
]. The assessment has demonstrated sensitivity and specificity measures
of 96% and 81%, respectively, suggesting a strong validity and reliability [
26
]. However,
advancements in the identification of objective TBI biomarkers will aid in diagnostic
precision while also improving treatment outcomes. This is especially important in a
military setting, where objective TBI biomarkers can inform quick and efficient decision-
making regarding a military personnel’s suitability to perform duties.

Brain Sci. 2022,12, 679 3 of 9
Advancements have been made towards understanding the impact of chronic low-
level blast exposure on military personnel; however, gaps remain. While many studies
have used eye-tracking technology to examine TBI, oculomotor assessment has not been
studied within a military population. Eye-tracking technology provides a unique non-
invasive and quantitative solution for examining the impact of low-level blast exposure
on military members. Therefore, the purpose of this study was to determine whether
long-term operators of low-level blast exposure or high-caliber weapons use displayed
oculomotor behaviors that differed from controls.
2. Materials and Methods
The total number of participants was 25 (between 32–53 years; M= 40.8, SD = 5.76). All
participants were male and members of elite Military units with lengths of service between
10–15 years (M = 13.7, SD = 1.2). The blast exposure group consisted of participants in the
‘Breachers Group’ (BC; n= 9) and ‘Gunners Group’ (GG; n = 9). Controls were in the ‘C
Group’ (CG; n= 7).
Participants were excluded from the study if they met any of the following pre-
screening conditions: neurological disorders (such as known concussion, Parkinson’s
disease); vision-related issues that prevented the successful calibration of all 9 points (such
as extreme tropias, phorias, static visual acuity greater than 20/400, cataracts, consumption
of drugs or alcohol within 24 h of testing) [8,27–29].
Participants were selected for the blast exposure group if they had met either of the
following conditions:
1.
Were trained specialty ‘Breachers’ within the Military Unit, where, by virtue of their
job description, to gain entry to locations, they used low-level explosives. These
operatives are therefore exposed to the risk of injury from debris, fragments or whole-
body translation.
2.
Were trained specialty ‘Gunners’ within the Military Unit, where, by virtue of their
job description, they were to operate high-caliber weapons.
Control group participants were Military personnel who were part of the elite units
but were not Breachers or Gunners. All participants did not wear additional protective
headgear beyond the standard helmet.
All participants provided informed consent to participate in this study in accordance
with IRB procedure. All testing was conducted by vision specialists (e.g., optometrists,
ophthalmologists) who had received and passed the RightEye training, education and
protocol procedures prior to testing. Additionally, all data was collected within a morning
session between 8–10 a.m.
Stimuli were presented via the RightEye tests on a Tobii I15 vision 15” monitor fitted
with a Tobii 90 Hz remote eye tracker and a Logitech (model Y-R0017) wireless keyboard
and mouse. The participants were seated in a stationary (non-wheeled) chair that could
not be adjusted in height. They sat in front of a desk in a quiet, private room. Participants’
heads were unconstrained. The accuracy of the Tobii eye tracker was 0.4
◦
within the
desired headbox of 32 cm
×
21 cm at 56 cm from the screen. For standardization of
testing, participants were asked to sit in front of the eye-tracking system at an exact
measured distance of 56 cm, which is the ideal positioning within the headbox range of the
eye tracker.
A Circular Smooth Pursuit (CSP) test was administered to all participants. Participants
were asked to ‘follow the dot, on the screen, as accurately as possible with their eyes as it
moved around in a circle’ (called Circular Smooth Pursuit (CSP)). The dot was 0.2 degrees
in diameter and moved at a speed of 25 degrees of visual angle per second. The tests were
taken with a black background with a white dot and lasted 20 s. The diameter of movement
of the CSP circle was 20 degrees.
Two metrics were used to examine the eye movements while conducting the CSP test
and included:

Brain Sci. 2022,12, 679 4 of 9
Fixation percentage: which accounts for the amount of time that the eye remains still
when it should be following the target. It is measured in milliseconds.
Smooth pursuit variance: the average distance that the eye deviates from the ideal
pathway. It is measured in millimeters.
The Sport Concussion Assessment Tool 2 was administered to all participants [
30
]. The
SCAT is used by healthcare professionals, and is a standardized tool for the acute evaluation
of suspected concussion [
25
]. The SCAT involves 22 questions asking the participant to rate
‘how they feel?’ on a scale of 0 (no symptom) to 1 or 2 (mild symptoms), 3 or 4 (moderate
symptoms), and 5 or 6 (severe symptoms).
The nature of the study was explained to the participants, and all participants provided
informed consent to participate. The study was conducted in accordance with the tenets of
the Declaration of Helsinki. The study protocols were approved by the Institutional Review
Board of East Carolina University. Following informed consent, participants were asked to
complete a pre-screening questionnaire and an acuity vision screening where they were
required to identify 4 shapes at a 4 mm diameter. If any of the pre-screening questions were
answered positively or if any of the vision screening shapes were not correctly identified,
the participant was excluded from the study.
Qualified participants who successfully passed the none-point calibration sequence
completed the eye-tracking test. Written instructions on the screen and animations were
provided before the test to demonstrate the appropriate testing behavior.
Once eye-tracking testing was complete, the participant completed the Sport Concus-
sion Assessment Tool 2 [30].
3. Results
3.1. Data Analysis
The differences in the groups (Control vs. Blast) were analyzed on clinically verified
data using JMP PRO 14.0 (SAS Institute; Cary, NC, USA). All variables were check for
multicollinearity (Table 1). The comparison was evaluated using one-way univariate
ANOVAs on the fixation stability measures, including: Visual Reaction Speed (RS), Fixation
Percentage, Eye-Target Velocity Error, Smooth Pursuit Variance, and Saccadic Velocity. The
alpha level was set at p< 0.05, and partial eta-squared (
ηp2
) was used to determine the
effect size. In addition, a series of receiver operating characteristic (ROC) curve analyses
were plotted for the fixation stability variables. A significant area under the curve (AUC)
with 95% confidence intervals (p< 0.05) was used to indicate the ability of each variable to
differentiate concussed participants from non-concussed ones. A stepwise multivariable
logistic regression model with an expert knowledge approach was used to assess the
relationship between Control and Blast groups and circular smooth pursuit variables:
Visual RS, Fixation Percentage, Eye-Target Velocity Error, Smooth Pursuit Variance, and
Saccadic Velocity. The total number of variables were reduced to avoid collinearity and to
include the variables with the most relevance to the research question. Global effect tests
were used to determine if a predictor was significant at α= 0.05.
Table 1. Strengths of the associations between the various eye-tracking parameters.
1 2 3 4
1. Visual Reaction Speed
2. Fixation Percentage −0.14
3. Smooth Pursuit Variance 0.32 0.46
4. Eye-Target Velocity Error −0.19 0.46 0.47
5. Saccadic Velocity 0.65 −0.08 −0.53 −0.63
3.2. Sport Concussion Assessment Tool 2
Table 2demonstrates a significant increase in SCAT
−
2 symptom severity and symptom
scores (p= 0.05). For the blast-exposed participants, there was an 18% increase in balance
problems; 20% increase in blurred vision; and approximately a 20% increase in headaches,

Brain Sci. 2022,12, 679 5 of 9
nausea, and light sensitivity. Other increases were seen in neck pain, falling asleep, and eye
strain. The remaining symptoms demonstrated little to no change pre and post exposure.
Table 2. SCAT-2 symptom scores pre and post impact.
Pre-SCAT-2 Symptom
Severity Score
Post-SCAT-2 Symptom
Severity Score
Control 0.40 (SD = 0.89) 0.63 (SD = 1.76)
Blast 20.36 (SD = 15.83) 32.87 (SD = 26.77)
3.3. Fixation Stability Measures Analysis
The ANOVA results for the Visual Reaction Speed and Circular Smooth Pursuit
Fixation Percentage demonstrated a significant main effect for Group [F(1, 24) = 5.336;
p= 0.03,
ηp2
= 0.181] and [F(1, 24) = 10.313; p< 0.001,
ηp2
= 0.301], respectively (see
Table 3). The ANOVA results for the Horizontal Smooth Pursuit Eye-Target Velocity
error demonstrated a significant main effect [F(1, 24) = 6.951; p= 0.04,
ηp2
= 0.22] and
Circular Smooth Pursuit: Smooth Pursuit Variance
[F(1, 24) = 4.069;
p= 0.049,
ηp2
= 0.144].
Further, the data demonstrated a significant effect for Vertical Saccades: Saccadic Velocity
[F(1, 24) = 5.53;
p= 0.027,
ηp2
= 0.187]; however, Targeting Displacement [F(1, 224) = 3.381;
p= 0.067,
ηp2
= 0.293] demonstrated a non-significant difference between the Control and
Blast groups.
Table 3. Mean and standard deviation for fixation stability variables.
Group (n) Visual RS Fixation Percentage Eye-Target
Velocity Error
Smooth Pursuit
Variance Saccadic Velocity
Control 350.13 (51.96) 3.63 (0.721) 17.34 (1.62) 6.88 (3.95) 65.10 (13.96)
Blast Exposure 397.94 (47.31) 4.62 (0.726) 18.61 (0.863) 12.46 (7.306) 51.79 (11.97)
3.4. Multivariable Logistic Regression
A stepwise multiple logistic regression analysis was conducted to evaluate how well
the criterion variable blast status predicted the visual function. The predictors were the five
smooth pursuit indices Visual RS, Fixation Percentage, Eye-Target Velocity Error, Smooth
Pursuit Variance, and Saccadic Velocity, while the criterion variable was Blast status. The
linear combination of Fixation Percentage and Saccadic Velocity was significantly related to
the TBI status,
χ2
= 14.109; p< 0.01, R
2
= 0.459. The other predictors did not significantly
contribute to the model and were removed (see Table 4). The final model accurately
predicted 84.6% of the TBI status, with a sensitivity of 94% and specificity of 63%.
Table 4. Estimated results for model coefficients.
B S.E. Wald df Sig. Exp(B)
Step 1 Fixation Percentage 2.091 0.933 5.021 1 0.025 8.09
Constant −7.786 3.759 4.291 1 0.038 0
Step 2
Fixation Percentage 3.161 1.519 4.33 1 0.037 23.594
Saccadic Velocity −0.169 0.098 2.989 1 0.084 0.844
Constant −2.385 4.658 0.262 1 0.609 0.092
Beta coefficient (B); Standard error (SE); Wald chi-squared test (Wald); Degrees of freedom (df); Statistical
significance (Sig); Odds ratio (Exp(B)).
Among the smooth pursuit parameters, the ROC curves were significant for the
Visual Reaction Speed, Fixation Percentage, and Smooth Pursuit Variance (see Table 5and
Figure 1). The remaining variables did not produce significant ROC curves and produced
low AUC scores.

Brain Sci. 2022,12, 679 6 of 9
Table 5. Summarization of outcomes for the ROC curve analysis.
Variable AUC S.E. p
Visual Reaction Speed
0.706 0.107 0.037
Fixation Percentage 0.785 0.098 0.023
Smooth Pursuit
Variance 0.785 0.104 0.023
Area under the curve (AUC); Standard error (S.E.); Probability value (p).
Brain Sci. 2022, 12, x FOR PEER REVIEW 6 of 9
Among the smooth pursuit parameters, the ROC curves were significant for the Visual
Reaction Speed, Fixation Percentage, and Smooth Pursuit Variance (see Table 5 and Figure 1).
The remaining variables did not produce significant ROC curves and produced low AUC
scores.
Table 5. Summarization of outcomes for the ROC curve analysis
Variable AUC S.E. p
Visual Reaction Speed 0.706 0.107 0.037
Fixation Percentage 0.785 0.098 0.023
Smooth Pursuit Variance 0.785 0.104 0.023
Area under the curve (AUC); Standard error (S.E.); Probability value (p).
Figure 1. ROC for CSP’s: (A) Visual Reaction Time Speed (AUC = 0.706); (B) Fixation Percentage
(AUC = 0.785); (C) Smooth Pursuit Variance (AUC = 0.785).
4. Discussion
The aim of this study was to determine whether long-term operators of low-level
blast exposure or high-caliber weapons use displayed oculomotor behaviors that differ
from controls. This was the first study to assess SPEM, saccades, and fixations within a
military population exposed to repeated low-level blasts and high-caliber weapon use.
The results revealed significant differences in SPEM, saccades, and fixations between the
blast exposure group and control group. There was no significant difference between
groups for targeting displacement. The results demonstrate that, in comparison to the con-
trol group, the blast exposure group’s eye movements were slower, stopped at more fre-
quent points when following a target, traveled further from the target in both speed and
direction, and showed higher rates of variation and inefficiency.
Oculomotor behavior has emerged as a sensitive biomarker for TBI, with an ability
to differentiate the diagnosis severity [8]. The CSP is the ability to follow a target around
Figure 1.
ROC for CSP’s: (
A
) Visual Reaction Time Speed (AUC = 0.706); (
B
) Fixation Percentage
(AUC = 0.785); (C) Smooth Pursuit Variance (AUC = 0.785).
4. Discussion
The aim of this study was to determine whether long-term operators of low-level
blast exposure or high-caliber weapons use displayed oculomotor behaviors that differ
from controls. This was the first study to assess SPEM, saccades, and fixations within a
military population exposed to repeated low-level blasts and high-caliber weapon use. The
results revealed significant differences in SPEM, saccades, and fixations between the blast
exposure group and control group. There was no significant difference between groups for
targeting displacement. The results demonstrate that, in comparison to the control group,
the blast exposure group’s eye movements were slower, stopped at more frequent points
when following a target, traveled further from the target in both speed and direction, and
showed higher rates of variation and inefficiency.
Oculomotor behavior has emerged as a sensitive biomarker for TBI, with an ability to
differentiate the diagnosis severity [
8
]. The CSP is the ability to follow a target around in
a circle while minimizing the amount of time that the eye remains still. Previous studies
have observed that individuals with a TBI have longer fixation percentages when engaging
in CSP [22]. The blast exposure group stopped moving their eyes significantly more often
when compared to the controls. This dysfunction is implicated in frontal lobe planning
and decision-making activities, only evident when a decision is required. These findings

Brain Sci. 2022,12, 679 7 of 9
contribute to a clearer understanding of the impact that chronic low-level blast exposure
has on the CSP fixation percentages of military personnel.
Variance in CSP is tracked in three segments of the pathway including middle,
left/right, and up/down. Intact vestibulo-ocular reflexes require efficient functioning
in a network of brain regions that spans the parietal and occipital lobes, premotor cortex,
and brainstem. Inefficient, high-variance eye movements are often indicative of TBI in these
regions [
7
]. The blast exposure groups had more than twice as much variation from the
target than the control group when engaged in smooth pursuit activity. This data provides
new insight into the relationship between repetitive low-level blast exposure and smooth
pursuit variance in military populations.
The results revealed that the blast exposure group was significantly slower than the
control group when engaged in saccadic eye movements. The slow saccadic eye movements
suggest that the blast exposure group had difficulty rapidly and efficiently moving their
eyes between targets. This is notable considering that multiple studies have observed a
relationship between impaired saccadic eye movements and TBI [
10
,
24
]. These findings
shed light on the potential for long-term low-level blast exposure to compromise the
saccadic eye movements of military special operators.
Collectively, the significant differences in SPEM, fixations, and saccades observed in
the blast exposure group are suggestive of TBI symptomology. This assertion is in line with
prior research suggesting that compromised saccades, fixations, and smooth pursuits are
implicated in TBI [
9
]. Unsurprisingly, poor oculomotor behavior in the blast exposure group
was correlated with a higher symptom severity on the SCAT in comparison to the controls.
Symptom severity was most evident through increased balance problems, blurred vision,
headaches, nausea, and light sensitivity. The accuracy of the model was 84.6%, which
suggests an excellent fit. Everything included, the data should be taken into account when
considering the impact of long-term low-level blast exposure and high-caliber weapon use
on the brain health of military personnel.
Practical implications converge on data that supports the refinement of military protec-
tive gear in addition to providing evidence of an objective TBI biomarker that can identify
those with effects from being exposed to low-level blasts and higher caliber weapons train-
ing. Notably, our findings build on existing evidence that suggests neurological alterations
may be implicated in chronic exposure to low-level blasts [
2
]. The military may benefit
from using this data to inform interventions that better protect the neurological health of
its service members.
Limitations of the current study include the fact that all 25 participants were male.
However, this factor was unavoidable given a current restriction on women serving within
the military units of the target population. A second limitation involves the CG’s exposure
to low-level blasts. While the CG participants were not trained as Breachers or Gunners,
most military personnel will have some exposure to blasts throughout their career, making
it challenging to control for this variable. Additionally, factors such as smoking, medication
use, and deployment history are unknown. It could be argued that the sample size is
small; however, the employed model differentiated the blast exposure group from the
control group and was substantiated by the overall statistical power of the test. Lastly, a
potential limitation is the use of a stepwise regression, which has been criticized by some
statisticians. A stepwise regression can be especially problematic with a large number of
predictor variables; however, our variable selection was based originally on an expertise
model and standard data reduction procedure. The final regression, albeit stepwise, does
align with our previous work and is supported by the outcome of the ROC analysis.
5. Conclusions
This study was the first to examine the impact of long-term low-level blast exposure
on the oculomotor behavior of military special operators. Future research should consider
splitting the blast exposure group in order to discern whether oculomotor behavior differs
between groups exposed to long-term low-level blasts and high-caliber weapon use. In

Brain Sci. 2022,12, 679 8 of 9
conclusion, the results of the study found that the oculomotor behavior of military spe-
cial operators exposed to long-term low-level blast exposure or high-caliber weapon use
differed significantly from controls in (a) CSP fixation percentages, (b) CSP variance, and
(c) saccadic eye movements, while poor oculomotor behavior was significantly correlated
with a higher symptom severity on the SCAT.
Author Contributions:
M.H., N.P.M., W.T.C. and F.R.C. contributed to the conceptualization, method-
ology, validation, formal analysis, and writing (original draft preparation and review and editing).
The supervision and project administration was completed by M.H. All authors have read and agreed
to the published version of the manuscript and have contributed substantially to the reported work.
Funding: This research received no external funding.
Institutional Review Board Statement:
The study was conducted in accordance with the Declaration
of Helsinki, and approved by the Institutional Review Board of East Carolina University (UMCIRB
18-000912, 1 June 2018).
Informed Consent Statement:
Informed consent was obtained from all subjects involved in
the study.
Data Availability Statement: The authors will make the data available should someone request it.
Conflicts of Interest:
Melissa Hunfalvay is a full-time employee with RightEye, LLC, and has relevant
affiliations and financial involvement. Nick Murray, William Creel and Frederick Robert Carrick have
no relevant affiliations or involvement.
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